Tunable RF capacitors fabricated on wafers have traditionally been made on substrates such as sapphire, silicon, alumina, and a host of other substrates. These substrates are a necessary evil to provide a mechanical medium on which to fabricate the capacitors, traditionally with standard semiconductor processes such as sputtering, etching, vapor deposition, photo-lithography, and so on. Once the processing is complete, the wafer is singulated by scribing or sawing and the individual circuits are packaged or used in bare die form within other circuits.
But each substrate has its pros and cons. Silicon is a very lossy substrate at RF frequencies, even high resistivity silicon, which “converts” back to lossy silicon when exposed to some of the higher temperature process steps needed in the fabrication of these parts. Sapphire is more expensive, and often is not compatible within silicon lines due to contamination issues. Alumina is limited in diameter and has issues of flatness. Ideally, no substrate is wanted, but the capacitors must be fabricated on something until they can be mechanically captured in the packaging material, typically plastic.
Consequently, there is a strong need for an invention which may solve the shortcomings set forth above.